The invention relates to telecommunications, and in particular, the invention relates to the transport of voice band signals with channel-associated signaling (CAS) through an Asynchronous Transfer Mode (ATM) system.
The well known DS-1 signal is a Time Division Multiplex (TDM) signal. The TDM signal can be structured into a 12 frame Superframe (SF) structure or a 24 frame Extended Superframe (ESF) structure. Both SF and ESF are well known in the art. FIG. 1 shows the composition of an ESF formatted TDM signal. The frame is comprised of 24 bytes from 24 separate voice channels and one overhead bit (not shown) for a total of 193 bits. The channels are byte-interleaved, meaning that a byte from channel one is followed by a byte from channel two, then a byte from channel three, and so on through channel 24. An ESF formatted TDM signal is comprised of 24 consecutive frames.
Channel-associated signaling is well known in the art. It is also known as robbed bit signaling. This is because the least significant bits of each channel in a given frame are "robbed" from the user and are used to carry signaling information. In an SF formatted TDM signal, bits are robbed in from each channel in frames 6 and 12. In an ESF formatted TDM signal, bits are robbed in frames 6, 12, 18, and 24. The signaling bits from the 6th frame are known as A bits and the signaling bits from the 12th frame are known as B bits. The signaling bits in the 18th and 24th frames may be called C and D bits, but they are typically a repetition of the A and B bits.
Asynchronous Transfer Mode (ATM) systems are also well known. ATM is a packet based system that uses fixed-length 53 byte (octet) cells. Each cell has a 5 octet header and a 48 octet payload. The functionality that converts signals to and from the ATM format is known as an ATM Adaption Layer (AAL). Standard AALs have been developed. The AAL used to transfer constant bit rates, such as uncompressed voice, is known as AAL 1. FIG. 2 depicts a cell created by AAL 1. The cell has a 5 octet ATM header and a one octet Segmentation and Reassembly Protocol Data Unit (SAR PDU) header in the first octet of the cell payload. This means that the cell now contains a five octet header, a one octet SAR PDU header, and 47 octets of SAR PDU payload. The SAR PDU header consists of a Convergence Sublayer Indication (CSI) bit, a Sequence Count Field (SCF) of 3 bits, a Cyclic Redundancy Check (CRC) field of 3 bits, and an Even Parity bit.
As depicted on FIG. 2, the prior art uses three bits for a sequence number. Three bits allow for an eight count (0-7). The CSI bit is sometimes used to hold clock synchronization information, but it may go unused. The remaining four bits in the SAR PDU header are used as an error check on the sequence number. Typically, the sequence number is used to detect lost or misplaced cells at the receiving end of the system
At present, a technique has been proposed to transport voice band channel-associated signaling through an ATM system. This proposed solution separates the A and B bits from the rest of the user information and transports these signaling bits in a special signaling octet. The signaling octet is placed after the 24 octets and comprises a 25th octet in the ATM payload.
This solution is lacking. Three disadvantages are:
1. Increased bandwidth requirement.
A 25th octet is added to carry the signaling information for every 24 octets of data. This means an additional 4% of bandwidth is required.
2. Performance degradation
Signaling bits are separated from the payload for transmission and are reinserted at the receiver. The reinsertion point is often in a different frame than the original frame the bits were taken from. This means that valid bits are replaced with the reinserted signaling bits. In effect, two bits have now been "robbed" to carry one signaling bit. This degrades performance.
3. Equipment complexity.
In general, the same equipment will have to handle both clear 64 kbit/s channels without signaling and voice band channels with signaling. If only signaling bits are to be removed from the signals, the clear 64 kbit/s channels without signaling will have to be processed differently than the 64 kbit/s voice channels with signaling. This increases the complexity of the equipment.